The present invention relates to the drive system of a crawler machine, and, more particularly, to the endless track and sprockets thereof.
In a conventional crawler drive system for a crane or other type machine, an endless track is received on spaced sprockets. The track is made of pivotally connected shoes, each of which has inwardly extending lugs. The sprockets (at least one of which is driven) have teeth which engage with the lugs on the shoe to drive the track and propel the machine in a forward or reverse direction.
It is known to have two laterally spaced lugs on each shoe with one lug longitudinally spaced forwardly of the other lug so that the lugs will be spaced apart far enough to minimize the collection of dirt between the lugs of a shoe. With this arrangement, one lug of a shoe is on one side of a vertical central plane extending longitudinally through the shoes, and substantially on either the front half of the shoe or on the rear half of the shoe (where front and rear may be considered with respect to the direction of movement of a shoe on the upper run of the track as the machine moves in a forwardly direction). The other lug of the shoe is on the other side of the vertical plane and substantially on the other half of the shoe. Thus, when viewed from inside the loop formed by the endless track, each shoe has two lugs positioned in diagonally opposite corners of the shoe.
Normally, the lugs of a particular shoe are engaged simultaneously by a pair of diagonally positioned teeth on the sprocket. The spacing between the pairs of teeth on the sprocket is slightly greater than the spacing between the pairs of lugs on successive shoes so that only one pair of lugs (on a particular shoe) are engaged by a pair of teeth on the sprocket at any given time. As the sprocket rotates, successive pairs of teeth engage the lugs of successive shoes, but at any given time only the one pair of lugs (of a single shoe) is engaged by a single pair of teeth on the sprocket. If the direction of rotation of the sprocket is reversed, to drive the machine in the opposite direction, the opposite faces of the teeth engage the opposite faces of the lugs. It should be noted that both faces of the teeth are inclined, as are both faces of the lugs, to facilitate a smooth meshing engagement between the sprocket teeth and the shoe lugs.
The difficulty of casting the shoes and/or sprockets with sufficient precision to assure complete engagement of the sprocket teeth with both lugs of a shoe creates misalignment which accelerates wear of the parts. For example, if one lug or tooth of a pair is not positioned precisely, the one or the other lug and tooth of the pair alone will be in driving engagement. If the single driving engagement between tooth and lug occurs at a face surface spaced a significant distance from the shoe pivotal axis under tension, the shoe engaged with the sprocket will tend to swing outwardly, because of the large moment to which the shoe is subjected. If the drive shoe is urged outwardly, the next shoe may not engage properly with the next pair of sprocket teeth, causing wear or even fracture of the shoe or sprocket.